Polyorganosiloxane compositions that can be cured to form elastomeric products are well known in the art. Modified organopolysiloxanes have been prepared in the prior art by polymerizing monomers containing aliphatic unsaturation in the presence of organopolysiloxanes containing terminal hydroxyl or hydrolyzable groups and free radical initiators.
Rufer et al., U.S. Pat. No. 4,536,553, disclose improving the flex life of silicone elastomers having olefinic unsaturation by mixing with the silicone elastomer a vinyl acetate ethylene copolymer in the presence of a free radical catalyst at an elevated temperature.
Jensen, U.S. Pat. No. 4,929,669, in an attempt to achieve low compression set, teaches the presence of a vinyl-specific peroxide in a liquid diorganopolysiloxane containing at least two silicon-bonded vinyl radicals per molecule which are curable to elastomers by a platinum catalyzed hydrosilation reaction. However, the combination tends to retard the platinum cure.
Kroupa et al., U.S. Pat. No. 4,500,659, disclose an extrudable, curable silicone elastomer composition prepared by reacting a reinforcing silica filler in a high viscosity marginally extrudable silicon elastomer composition with a liquid fluorinated polyorganosiloxane.
Special mention is made of Nelson, U.S. Pat. No. 3,284,406 which discloses a mixture consisting essentially of (1) a polysiloxane of the formula ##STR1## in which R and R' are phenyl or methyl and at least 80 mole percent of the R' groups are methyl, said siloxane (1) having a viscosity of from 500 to 500,000 centipoise inclusive at 25.degree. C.; (2) from 5 to 50 percent by weight based on the weight of (1) and (2) of a copolymer of SiO.sub.2, Me.sub.3 SiO.sub.0.5 and Me.sub.2 ViSiO.sub.0.5 siloxane units in which copolymer there is from 1.5 to 3.5 percent by weight vinyl groups based on the weight of (2) and in which copolymer (2) the ratio of total Me.sub.3 SiO.sub.0.5 and Me.sub.2 ViSi.sub.0.5 to SiO.sub.2 units is from 0.6:1 to 1:1; (3) a compound compatible with (1) and (2) which is a siloxane containing from 0.1 to 1.7 percent by weight silicon bonded hydrogen atoms the remaining valences in (3) being satisfied by phenyl or methyl radicals, there being at least three silicon-bonded hydrogen atoms per molecule, and in (3) any hydrocarbon radicals attached to an SiH silicon being essentially all methyl radicals, the amount of (3) being such that there is from 0.75 mol of SiH per mol of vinyl radicals in (1) and (2) to 1.5 mol of SiH per mol of vinyl radicals in (1) and (2); and (4) a platinum catalyst.
Jeram et al., U.S. Pat. No. 3,884,866 disclose an organopolysiloxane composition comprising a first component comprising a mixture of two vinyl-containing polysiloxanes or blends of vinyl-containing polysiloxanes one of which is a high viscosity polysiloxane and the other of which is a low viscosity polysiloxane, a filler and an effective amount of a platinum catalyst; and a second component comprising a hydrogen-containing silane or polysiloxane.
Jeram et al., U.S. Pat. No. 4,340,709 disclose an addition curing silicone composition with a low viscosity in the uncured state and high physical strength in the cured state comprising a vinyl-containing diorganopolysiloxane polymer, a platinum catalyst, a hydride cross-linking agent and a linear hydride coupler in which the viscosity of the linear hydride coupler varies from 1 to 1,000 centipoise at 25.degree. C.
However, the prior art silicone elastomers, while providing good solvent resistance and other physical properties suffer from such drawbacks as poor heat aging and high compression set characteristics making them unsuitable for use in many applications, such as gaskets. It would therefore represent a notable advance in the state of the art if a silicone composition could be provided having excellent heat stability characteristics, rendering the composition useful where heat stability is important such as in gasketing, coatings and potting applications as measured by oven aging and percent compression set. Unexpectedly, the compositions of the present invention satisfy such a long felt need, exhibiting excellent low compression set and heat aging characteristics. Such surprising results are shown in the working examples hereinbelow.